Bacon also listed what he called the idols (false images) of the mind. He described these as things which obstructed the path of correct scientific reasoning.

Idols of the Tribe (Idola tribus): This is humans' tendency to perceive more order and regularity in systems than truly exists, and is due to people following their preconceived ideas about things. Idols of the Cave (Idola specus): This is due to individuals' personal weaknesses in reasoning due to particular personalities, likes and dislikes. Idols of the Marketplace (Idola fori): This is due to confusions in the use of language and taking some words in science to have a different meaning than their common usage. Idols of the Theatre (Idola theatri): This is the following of academic dogma and not asking questions about the world.

These four fallacies are sometimes compared to a similar list in the first part of Roger Bacon's Opus Majus which, although it was much older, had not been printed in Francis Bacon's time.